The coking plant is an essential part of iron and steel industry. However, its production activities, such as coal coking, coal-gas purification and recovery of coking byproducts, engender a large quantity of wastewater, namely, coking wastewater. Coking wastewater contains very complex components, particularly phenols, and its exact composition varies in accordance with raw coal's nature, carbonization temperature and procedures and methods adopted in recovering coking byproducts. Apart from phenols and phenolic derivatives, other major organic components in wastewater are aliphatic compounds, heterocyclic compounds and multicyclic aromatic compounds. It also contains various inorganic compounds such as ammonia salts, thiocyanides, sulfides, and cyanides. Hitherto, there has been no satisfactory way in China to purify coking wastewater and make it meet national legal discharge standards. The major reason is that the existing techniques present undesirable effect in treating coking wastewater due to the complex and characteristic pollutants contained in it. Currently, the most widely-adopted method in treating coking wastewater is active sludge process, which does present certain effect. However, since phenolic, cyanide compounds and ammonia nitrogen contained in coking wastewater show great resistance against decomposition effect of this process and the aerobic microorganisms in the sludge may suffer shock or even death when the concentration of these toxic components turns too high. Therefore, this method presents unstable treatment effect and the quality of wastewater so treated is still lower than the highest standard set for safe discharge.
Currently, there are many techniques and methods for treating coking wastewater produced by iron and steel plants. As far as lowering down COD and NH3 is concerned, there are two often used processes: anaerobic-anoxic-oxic (A1-A2-O) process and anoxic-oxic (A-0) process. Due to complex components in the wastewater, particularly high concentration of toxic and recalcitrant organic chemicals therein, coking wastewater treated with either of these two processes hardly meets discharge standards in terms of COD, phenols, cyanides and chromaticity, even though BOD5 may satisfy the requirements. Some other newly-developed advanced treatment (tertiary treatment) techniques may be able to obtain safe discharge, but they meanwhile present many defects, such as long hydraulic retention time, consumption of active carbon grains and coagulants, or renewal of carbon source, which consequently increases operation expenditure and even endangers stability of the quality of discharged water.
In consideration of a large amount of organic chemicals, particularly those recalcitrant against bio-decomposition, in coking wastewater and high cost for implementing tertiary treatment system, it is necessary to find a new, highly efficient pre-treatment method in order to take the place of the existing tertiary treatment techniques. In this pre-treatment stage, most of organic chemicals should be decomposed and toxic pollutants should be dramatically reduced, which in turn ensures high effectiveness of later routine biochemical treatment (secondary treatment) and eventually obtains stable and safe discharge.
There are many techniques designated for pre-treatment of coking wastewater, among which many advanced oxidation methods are adopted, including supercritical water oxidation, electrochemical oxidation, Fenton oxidation, photocatalytic oxidation/microelectrolysis, ultrasonic treatment and pulsed high-voltage discharge treatment. Most of techniques are still in their laboratory stage and show distinctive effect on removing one or two pollution indices. However, they fail to obtain comprehensive removal of most pollution indices despite their high operation expenditure. Therefore, it takes a long time before they can be applied for practical utilization.
Wu Keming from Wuhan University of Science and Technology (Chemical Engineers Journal. 116.5) discloses an electrolytic flocculation technique for treating coking wastewater. Both cathode and anode of the electrolyzer are made of aluminum plates and DC electricity generated by a transistor current to stabilizer is used as electrolytic power supply. This method shows excellent effect in solving turbidity of coking wastewater but poor performance in reducing such pollution indices as chromaticity and COD.
Ma Qian, et, al. from College of Life Science and Technology of Tongji University (Techniques and Equipment for Environmental Pollution Control. 6.8 (2005)) discloses another electrolytic flocculation technique in which aluminum plates are used as the cathode and anode of the electrolyzer, and pure DC electricity supply is used in treating coking water that has experienced biochemical treatment yet still fails to meet safe discharge standard (COD=300-500 mg/L). The outcome of experiment shows that the discoloration rate of coking water reaches 93.6%. Though COD removal rate is up to 61.78%, its concentration in treated wastewater is still higher than required standard, which means further advanced treatment has to be applied before the wastewater meets the highest discharge standard stipulated by Chinese government.
He Zhenghao, et, al. from Huazhong University of Science and Technology (High Voltage Engineering. 29.4) discloses a new technique for pre-treatment of coking wastewater in which the technology of nanosecond pulsed corona discharge is adopted to pretreat high-concentration coking wastewater in spraying form. This technique can remove more than 90% of cyanides and 70% of phenols. However, its performance in removing ammonia nitrogen and COD is disappointing. In addition, analysis of its outcome shows that this technique is able to improve the biodegradability of coking wastewater.
In summary, the composition of coking wastewater is very complex. It contains not only such decomposable chemicals as phenols and phenolic derivatives, but other recalcitrant chemicals like multicyclic compounds and heterocyclic aromatic compounds as well. Though it is a cost-effective way utilizing anaerobic and oxic microorganisms in coking wastewater treatment, the wastewater treated in this way hardly satisfies increasingly high environmental requirements as far as such pollution indices as CODCr, chromaticity, and NH3—N are concerned.
Though there are many techniques, both in China and other countries, concerning pretreatment of coking wastewater, most techniques used in current China embody many defects like high cost in such pretreatment processes as distilling ammonia and extraction as well as inefficient reuse of recovered phenols, which may otherwise offset part of treatment cost. As to simple, routine electrolytic oxidation, it may be effective in removing one or two pollution indices; but its poor comprehensive performance makes it hard to be adopted in large scale application. The pre-treatment system of coking wastewater for practical application calls for high efficiency and significant effect in removing all pollutants; it should also enjoy such favorable features as high working stability, reasonable investment and low operation expenditure.